What is the waterfall model?

Waterfall model: definition and phases

The waterfall model describes software projects as a chain of phases in which the results of one phase form the basis of the next—like water flowing down step by step. It is often traced back to a 1970 paper by Winston Royce, in which the author himself warned of the risks of a purely sequential process.

Typical phases are requirements analysis with a requirements and a functional specification, the design of architecture and interface, implementation, testing, acceptance and rollout, and operation and maintenance. At the end of each phase comes a sign-off, usually with a document as the result.

Strengths and weaknesses

Strengths include the clear structure, a scope fixed early, firm milestones, complete documentation and good suitability for fixed-price offers. When all requirements are known and stable, the model is easy to plan and manage, and everyone knows which phase the project is in.

The weakness: feedback comes late. Users see the software only during testing or at acceptance, and misunderstandings from the early phase then become expensive. If requirements change during the project, which happens often, phases that were already completed have to be reopened, and the later an error is found, the more costly it is to fix.

• Requirements are fully known and hardly change
• Legal or contractual documentation obligations apply
• Deadline and price are fixed in advance
• Interfaces to existing systems are clearly specified
• Hardware-related development, where later changes are very expensive

Real-world example: an interface between inventory management and accounting

A mid-sized company wants to link its inventory management with its accounting. Both systems are known, the data fields are documented, and functions and flow are settled. A waterfall approach fits here: the functional specification is agreed, the interface is developed, tested and put into operation according to clearly defined acceptance criteria.

It would be different for a new customer platform whose functions only emerge in use. There, a rigid specification would miss the actual need, and an agile approach would be the better choice, because users can contribute early and priorities can be adjusted.

Hybrid forms and imple­menta­tion

In practice, many projects work in a hybrid way: the framework of budget, deadlines and acceptance is planned in the traditional way, while implementation takes place in short cycles with interim demos. This combines predictability with flexibility. Typical mistakes in pure waterfall are requirements phases that run too long, tests that come too late and users who are not involved.

Nano Concepts helps you choose the right approach and records requirements with you so that quote, implementation and acceptance fit together—whether traditional, agile or mixed. What matters is that you get something visible early and that changes remain possible in a controlled way.